This invention relates to dispensing apparatus and, more particularly, to apparatus for dispensing liquid onto a workpiece and for preventing further drops from falling on the workpiece.
In the processing of materials for the ultimate manufacture of electronic circuit components, one of the steps required may be the dispensing of a fixed amount of a certain type of liquid onto the surface of a workpiece. For example, in the manufacture of integrated circuits, a dopant material may be dispensed onto a silicon wafer for subsequent fusion in a furnace. Additionally, in the manufacture of printed circuit components, a photoresist material may be dispensed onto a substrate preliminary to photoetching of the substrate. In both of these examples, a fixed amount of a dispensed liquid is deposited upon the workpiece surface and is then spread evenly thereon, for example by spinning the workpiece at a high rate of speed. The exact amount of dispensed liquid is not particularly critical, however it is critical that after the dispensing no further drops of the liquid may be dispensed thereon.
Whenever a liquid is dispensed through an orifice, and a valve supplying the liquid to the orifice is closed, the liquid of its own weight forms a final drop. In the situations described above, this final drop would cause the workpiece to be rejected because if the final drop comes down after the workpiece begins spinning, it creates a discontinuity in the surface of the workpiece and if the spinning is delayed until after the final drop comes down, due to the volatility of the solvents in the liquids, there would be too much drying of the liquid before the spinning. Therefore, this final drop must be prevented.
To prevent the dispensing of this final, late drop, the prior art has devised apparatus for dispensing a fixed amount of liquid through a nozzle and then sucking back the liquid that was left at the end of the nozzle in order to prevent the formation of the final drop. This requires a complicated series of diaphragms, pumps, and the like, which is relatively expensive.
It is therefore an object of this invention to provide apparatus for dispensing a fixed amount of liquid onto a workpiece.
It is another object of this invention to provide such apparatus wherein after the fixed amount of liquid is dispensed, no further amount of the liquid falls on the workpiece.
It is yet another object of this invention to provide such apparatus which is simple in design and economical in cost.